It is known to adjust the electrical characteristics of an SRAM cell by changing the body potential of the well of a transistor etc., which is a component of the SRAM cell, or the potential of the word line in order to suppress the influence of manufacturing variations of the semiconductor device on which the SRAM cell is mounted. It is possible to measure the electrical characteristics of the transistor, such as the on-current and the threshold voltage, used when adjusting the electrical characteristics of the SRAM cell as analog signals by using an electric probe, etc. However, in such measurement, analog signals are measured, and therefore, the measurement is vulnerable to various kinds of measurement noises and it is not easy to perform measurement with high precision.
On the other hand, it is known to use a ring oscillator in order to evaluate the speed characteristic of various kinds of elements to be mounted on the semiconductor device. A ring oscillator is formed by ring-connecting a single or a plurality of non-inversion elements and an odd number of inversion elements.
Further, there is known a semiconductor device having a logic part forming a logic circuit and a memory part formed by a plurality of SRAM cells laid out in the form of an array. In such a semiconductor device, various kinds of logic elements of the transistors laid out in the logic part are laid out based on the same layout wiring rules. On the other hand, the SRAM cells laid out in the memory part are laid out, in many cases, based on layout wiring rules different from those of the transistors used in the logic part forming the logic circuit. Since an SRAM cell generally has a six-transistor configuration, layout wiring rules in which the layout intervals are narrowed as much as possible are adopted in order to prevent the layout wiring area from increasing.
It is known to use a ring oscillator formed by ring-connecting a plurality of SRAM cells in order to evaluate the speed of the SRAM cells to be laid out based on layout wiring rules different from those of the logic part. For example, a ring oscillator formed by ring-connecting a plurality of SRAM cells in which the gate and the drain of a pair of transmission transistors are connected to each other is used. By connecting the output terminal of an inverter element inside the SRAM cell and the drain of the transmission transistor of the SRAM cell, delay circuits configured to output an inverted signal are ring-connected.